With the recent technological advances in regard to integration and miniaturization, and by virtue of the development of low bitrate and very low consumption wireless communication technologies, a new application field has emerged by the name of wireless body networks or BAN, the acronym standing for “Body Area Networks”. Hereinafter the terms “body network” and “BAN” will be employed interchangeably. In this type of application, wireless transmission and/or reception elements are intended to form a network on or in very close proximity to a human body. Known applications of these networks are notably the “exploded terminal” (comprising screens, keyboards, and earpieces, not co-located), sports apparatus (cardio-frequency meter, watch, pedometer on a shoe) or medical apparatus (nomadic monitoring of cardiac, cerebral and muscular activity, for example). The needs in respect of autonomous, intelligent wireless body networks capable of responding to the needs of emerging applications, in fields as varied as security, health, sport and mass entertainment, are therefore growing regularly.
Furthermore, the locating of the wireless devices constituting a BAN is of interest in numerous applications such as:                navigation of groups of people moving around indoors and geo-located services;        motion capture, for example, for the tracking of sports movements or for entertainment and games applications;        posture detection, for example, for rehabilitation, the tracking of vulnerable or elderly people, and the surveillance of people operating in a disaster-stricken environment, for example firemen operating inside a blazing building.        
Hence, it is desired to improve the accuracy of positioning of the nodes attached to a mobile body and/or to limit the number of radio linkups necessary in a context of cooperative location within groups of mobile wireless body networks.
The issue of location and positioning has already been studied fairly widely, notably in the context of ad hoc networks and of sensors. It will be possible notably, in this regard, to consult the publication by the authors N. Patwari, J. N. Ash, S. Kyperountas, A. O. Hero, R. L. Moses, and N. S. Correal, “Locating the nodes: cooperative localization in wireless sensor networks,” Signal Processing Magazine, IEEE, vol. 22, July 2005, pp. 54-69, document referenced [DOC 1] subsequently. The problem of location within the framework of wireless body networks, networks commonly designated by the initials WBANs, has been explored but the proposed solutions do not utilize the characteristics of the multi-path channel, and are limited notably to basic metrics for estimating the distances between the nodes. The following publications may be cited, notably in respect of the techniques utilizing the RSSI (for “Receive Signal Strength Indicator”) metric:                C. P. Figueiredo, N. S. Dias, and P. M. Mendes, “3D localization for biomedical wireless sensor networks using a microantenna”, Wireless Technology, 2008. EuWiT 2008. European Conference on, 2008, pp. 45-48, document referenced [DOC 2] subsequently;        C. Guo, J. Wang, R. V. Prasad, and M. Jacobsson, “Improving the Accuracy of Person Localization with Body Area Sensor Networks: An Experimental Study,” Consumer Communications and Networking Conference, 2009. CCNC 2009. 6th IEEE, 2009, pp. 1-5, document referenced [DOC 3] subsequently;        H. Ren, M. Q. Meng, and L. Xu, “Indoor Patient Position Estimation Using Particle Filtering and Wireless Body Area Networks,” Engineering in Medicine and Biology Society, 2007. EMBS 2007. 29th Annual International Conference of the IEEE, 2007, pp. 2277-2280, document referenced [DOC 4] subsequently;        Choi, “System, Apparatus and Method for Keeping a Person Under Surveillance”, international patent application referenced under the publication number WO 2008/143379 A1, on 27 Nov. 2008, document referenced [DOC 14] subsequently;        H. Baldus et Al., “Method for Positioning of Wireless Medical Devices with Short-Range Radio Frequency Technology”, international patent application referenced under the publication number WO 2005/096568 A1, on 11 Mar. 2005, document referenced [DOC 15] subsequently;        B. E. Funk et Al., “Method and System for Locating and Monitoring First Responders”, American patent application referenced under the publication number US 2008/0077326 A1, on 27 Mar. 2008, document referenced [DOC 16] subsequently;and in respect of the techniques utilizing the DOA metric:        F. Chiti, R. Fantacci, F. Archetti, E. Messina, and D. Toscani, “An Integrated Communications Framework for Context Aware Continuous Monitoring with Body Sensor Networks”, IEEE Journal on Selected Areas in Communications, vol. 27, No. 4, May 2009, document referenced [DOC 7] subsequently;a technique utilizing the TOA or TDOA metric on the basis of the direct paths:        Y. Caritu et Al., “Motion Capture Device and Associated Method”, international patent application referenced under the publication number WO 2007/093641 A2, 2007, document referenced [DOC 18] subsequently;and a technique utilizing ultra-sounds:        G. Vannucci et Al., “System and Method for Motion Capture in Natural Environments”, American patent application referenced under the publication number US 2008/0223131 A1, on 18 Sep. 2008, document referenced [DOC 17] subsequently.        
In radio, the metric which is conventionally utilized is the time of flight, from which a distance is directly gleaned by knowing the speed of electromagnetic waves in air. Under real conditions, the measured time of flight is affected by an uncertainty related to several phenomena such as thermal agitation noise, multi-path phenomena and situations of obstruction of the radio linkups, these situations also being referred to as NLOS situation, standing for “Non-Line-Of-Sight”.
It is notably to address these constraints that UWB (“Ultra Wide Band”) technology has been studied in a locating context, this technology benefiting from intrinsic properties of good temporal resolution, be it in operation of RADAR type or of cooperative type (as in a conventional context of digital communications). This technology, which has taken hold as an alternative physical layer for wireless sensor networks (WSNs) with low bitrate or LDR (for Low Data Rate) and with low electrical consumption or ULP (for Ultra Low Power) relies on the transmission of signals for which the ratio between the width of the band at −10 dB and the central frequency is greater than 20%, or making use of an absolute band of greater than 500 MHz. In particular, these ultra wideband impulse communication systems provide for the transmission of coded and/or modulated trains of brief pulses. In the time domain, the high separating power of such signals and of the systems associated with them, directly attributable to the width of the spectral band occupied, permits the resolution of dense multi-path profiles on reception.
FIG. 1a, taken from the publication W. Yang et Al., “Time-Domain Investigating Path Loss Characteristics of UWB Signal in Indoor line-of-sight Environment”, The 2nd IEEE International Conference on Wireless Communications, Networking and Mobile Computing, 2006, illustrates a scenario of UWB radio transmission between a transmitter 101 and a receiver 102 indoors, in this instance in a room with ceiling 111, floor 112, and walls 113, 114, 115. In the course of its propagation, the radio wave undergoes various types of electromagnetic interactions with the environment, including among them reflections on reflecting surfaces and/or diffractions, on sharp edges. Several copies 121, 122, 123, 124, 125 of the transmitted signal are then received at the level of the receiver 102, with different temporal and spatial characteristics. This phenomenon is better known by the name multi-path propagation. One of the benefits of UWB systems is that the good temporal resolution of the receiver makes it possible to distinguish the various copies of a received signal, and to be able to restore a sufficiently accurate image of the multi-path propagation channel (this image also being called the impulse response of the channel or CIR for “Channel Impulse Response”).
FIG. 1b, taken from the publication W. W. Ji et Al., “A Fuzzy Logic-Based Ranging Technique for UWB Radio Link”, IEEE WICOM 2006, illustrates an exemplary impulse response of a multi-path propagation channel. As ordinate is represented the amplitude of the response and as abscissa the time. The estimated component corresponding to the presumed direct path 131 does not necessarily correspond to the first real path actually received 132—since a pulse corresponding to an occurrence of noise and whose amplitude exceeds the detection threshold may be confused with the reception of a wave path—and/or also does not necessarily correspond to the path having the largest amplitude 133. Such is notably the case when the radio communication linkups between the nodes of a BAN are in a situation of obstruction of the communication linkups (NLOS), as shown by FIG. 1c. 
In contradistinction to a data transmission context for example, where frequently the receiver seeks to synchronize itself on the strongest path (or a group of strongest paths), the location applications require the receiver to very accurately determine the component corresponding to the propagation of the first path received, presumed that of the direct path, in order to accurately measure the time of arrival, and thus to deduce therefrom the distance between the transmitter and the receiver. The following publications may notably be cited as regards the study of the characteristics of the UWB channel:                T. Zasowski et Al., “Propagation Effects in UWB Body Area Networks”, IEEE ICU 2005, [DOC 19];        R. Fort et Al., “Ultra-Wideband Channel Model for Communication Around the Human Body”, IEEE Journal on Selected Areas in Communications, vol. 24, N. 4, pp. 927-933, 2006, [DOC 20];        Fort, et Al., “Ultra Wide-band Body Area Channel Model”, IEEE ICC 2005, pp. 2840-2844, vol. 4, [DOC 29].        
FIG. 2 illustrates, through a first example gleaned from the document DOC 19 cited above, the energy contribution relating to two groups of different paths 201, 202 for an environment of “desk” type. The first group 201, which arrives in less than 1 ns at the receiver node, can be associated with the direct path. The second group 202, appearing after 6 ns, is for its part associated with a path reflected on a desk situated in proximity. The remainder of the energy arrives thereafter in a more diffuse manner.
FIG. 3 illustrates, through a second example gleaned from the document DOC 20 cited above, the richness of a multi-path profile on reception, and therefore of the information that it is possible to glean from the environment, and in particular simple reflections, which reflections give rise to noteworthy and significant echoes. By way of example, after the first “direct” path 301 occurring after 2-3 ns, a first strong reflection 302 is noted fairly distinctly around 7 to 8 ns, corresponding to a reflection on the ground (in the example with an equivalent distance traveled of close to 2.5 m).
In a last example illustrated in FIG. 4 and gleaned from the document DOC 29 cited above, the impulse response of a multi-path propagation channel is represented for a signal transmission between a node placed in the front of a human body and a node situated on a fixed anchor in proximity to the body. A first group of received signals 411 corresponds to the diffraction of the wave around the torso of the body and a second group 412 is due to the reflection on the ground.
The wireless body network locating solutions proposed in the prior art do not utilize ultra-wideband communication technology together with the diversity of the multi-path profile received (cf. the publications referenced above in this preamble DOC 2 to DOC 17).
However, certain techniques have been proposed for location and/or mapping based on UWB systems, either by utilizing solely the information related to the direct paths, or by taking into account at one and the same time the direct paths and a set of secondary paths.
The techniques utilizing solely the information related to the direct paths are firstly presented. A technique presented in (cf. DOC 18 cited above), pertains to an autonomous system for determining information representative of the motion of an articulated chain comprising at least two solid elements and at least one articulation linking said two elements. However, this technique does not utilize the multi-path diversity of the UWB channel and is based solely on the estimation of the direct paths between communication nodes. Moreover, each mobile element consists of a set of nodes which are placed at very specific sites so as to determine the motion of articulations.
Another procedure, described in the international patent application referenced under the publication number WO 2007/067821 A2, on 14 Jun. 2007, [DOC 22], is a positioning technique relying on UWB radio technology and the time of arrival (or TOA for Time-Of-Flight) metric. The invention implements multiple measurements of distances in time and in space so as to improve location accuracy. However, this procedure does not utilize the multi-path diversity of the UWB channel and is based solely on the estimation of the direct paths. Moreover, each mobile element (or terminal) carries just a single radio communication node.
The technique presented in the American patent published under the number U.S. Pat. No. 7,397,379 B2 on 8 Jul. 2008, [DOC 23], takes the form of a system making it possible to locate mobile people, indoors. This technique relies on UWB radio technology, where each person is associated with an ultra wideband radio node making it possible to carry out measurements of distances in relation to anchors or other people within communication range. However, this technique does not utilize the information relating to the secondary paths (or those arising from reflections). Moreover, each mobile element (person, etc.) carries just a single radio communication node.
Certain known locating techniques utilize secondary paths by means of a statistical or stochastic approach. In this regard may be cited:                M. Najar, J. Vidal, “Kalman Tracking for Mobile Location in NLOS Situations”, in Proc. IEEE PIMRC'03, vol. 3, pp. 2203-2207, September 2003 [DOC 36];        B. Denis, L. Ouvry, B. Uguen, F. Tchoffo-Talom, “Advanced Bayesian Filtering Techniques for UWB Tracking Systems in Indoor Environments”, in Proc. IEEE International Conference on Ultra-wideband, pp. 638-643, Zurich, September 2005 [DOC 37].        
These techniques utilize the estimation of the times of arrival (TOA) corresponding to secondary paths (due to the obstruction of the direct radio linkups not in direct visibility or NLOS), thus inducing a bias in the measurements. This bias is generally estimated in random walk form by means of filtering procedures, in order to improve the accuracy of location.
However, this locating procedure does not utilize any relation between the times of arrival of the secondary paths and the position of the communication nodes. Moreover, each mobile element carries just a single radio communication node involved in distance measurements; stated otherwise, either the mobile element carries just a single wireless node, or the mobile element carries several nodes but only a single of these nodes is involved in the distance measurements.
Certain hybrid or semi-deterministic approaches have also been explored, such as for example that published by J. Youssef, B. Denis, C. Godin, S. Lesecq, “Enhanced UWB Indoor Tracking through NLOS TOA Bias Estimation”, IEEE Global Communications Conference 2008 (IEEE GLOBECOM'08), New Orleans, USA, November-December 2008, [DOC 43]. These are based, for example, on a deterministic modeling of the angular dependencies of the bias introduced by the mobility and the presence of multi-paths related to the NLOS-type configuration of the UWB channel. However, these locating procedures do not utilize any relation between the times of arrival of the secondary paths and the position of the communication nodes. Moreover, each mobile element carries just a single radio communication node involved in distance measurements.
Other location techniques utilize secondary paths by means of a deterministic approach in order to express an explicit relation between the times of arrival of the secondary paths and the position of the nodes. For example, in:                V. La Tosa, B. Denis, B. Uguen, “Maximum Averaged Likelihood Estimation Tree for Anchor-Less Localization Exploiting IR-UWB Multipaths”, in Proc. IEEE VTC-Spring'10, Taipei, May 2010, [DOC 31]; and        B. Denis, V. La Tosa, B. Uguen, F. Tchoffo-Talom, “METHOD AND SYSTEM FOR AIDING ENVIRONMENTAL CHARACTERIZATION BY ULTRA-WIDEBAND RADIOFREQUENCY SIGNALS”, international patent application referenced under the publication number WO/2009/077510, June 2009, [DOC 33].        
The times of arrival of the paths arising from simple reflections on the walls are utilized in part to retrieve the dimensions of rooms and the relative positions of a pair of low-bitrate UWB radio nodes. A similar approach, developed by W. Guo, N. P. Filer, “2.5D Indoor Mapping and Location Sensing using an Impulse Radio Network”, in Proc. IRR Seminar on Ultra Widehand Systems, Technologies and Applications 2006, pp. 211-215, London, April 2006, [0024], termed radio mapping inside buildings (or “indoor mapping”), provides for the positioning of a node or of a pair of nodes in a room with no reference point, that is to say with no node knowing its position a priori. This approach is based mainly on a geometric and deterministic interpretation of the times of arrival of the significant echoes obtained on reception. These echoes are presumed to arise from a simple reflection or from multiple reflections on the walls. Elementary mathematical relations make it possible to establish a link between the arrival time patterns obtained and the relative positions of the nodes in a room (for example the position with respect to the walls). It should be noted that this procedure allows the positioning of a single node (via the probing of its own channel in mono-static mode, that is to say with a transmitter and a receiver situated on one and the same node and operating according to the principle of radar: the node transmits a signal which is backscattered by the environment and then received by this same node), or of a pair of nodes (via the probing of the channel between these two nodes in bi-static mode, that is to say by transmission of a signal between a transmitter and a receiver situated on remote nodes). However, this procedure determines only a relative positioning, of a single node, or of a pair of nodes; furthermore, it lacks accuracy. Moreover, this procedure does not make use of any means for bounding the proximity of the nodes in relation to the reflecting surfaces (this being different from the BANs where the nodes are necessarily between 0 m and 2 m from the ground for BANs implanted on human bodies). Moreover, each mobile element carries just a single radio communication node.
Another technique, termed space regionalization (or “Georegioning”), makes it possible to establish a coarse positioning of the nodes by utilizing the whole of the multi-path profile received. This technique has notably been developed in the following publications:                F. Althaus, F. Troesch and A. Wittneben, “UWB Geo-Regioning in Rich Multipath Environment”, IEEE 2005, [DOC 38];        F. Althaus, F. Troesch, A. Wittneben, “UWB Geo-Regioning in Rich Multipath Environment”, VTC fall 2005, [DOC 39];        Frank Althaus, Florian Troesch and Armin Wittneben, Geo-Regioning for UWB Networks, IST FA, [DOC 40];        C. Steiner, F. Althaus, A. Wittneben, “On the Performance of UWB Geo-Regioning”, SPAWC 2006, [DOC 41];        
For example, within an asynchronous UWB network, for a given position of the transmitter node and various positions of the receiver, the profiles of power received as a function of time are initially collected, these profiles sometimes being designated by the initials PDP for “Power Delay Profile”. The average of a subset of the measurements gathered for one and the same region of space (or APDP for “Average Power Delay Profile”) is then associated with this same region in the guise of radio UWB signature. This first phase constitutes a learning phase. Subsequently, for the coarse positioning of a node abreast in the network, each new measurement of PDP at the level of the receiver is contrasted with the average APDP obtained for each region, and a decision is taken as regards the geographical membership of the node. The decision rule, such as it is presented in the document DOC 38 cited above, is based on the maximization of the likelihood of the new observation conditioned upon the expected mean profile for each region (ML for “Maximum Likelihood”). However, this procedure requires a calibration or learning step usually requiring an intensive campaign of measurements. Furthermore, the positioning is deliberately inaccurate, since a node is positioned as belonging to a certain region of space.
Finally, other techniques for recognizing radio imprints or patterns, also called “fingerprinting” or “pattern recognition”, also make it possible to exploit the whole of the multi-path profile received. These techniques contrast current measurements, derived from the observation of the UWB signals received and carried out as dictated by the movement of a node to be positioned, with measurements or simulations carried out beforehand. These are constituents of a database, for which a correspondence is ensured with the exact coordinates of the nodes involved in the radio links. For example, the technique published by B. Denis, “Exploitation des Capacités de Radiolocalisation des Transmissions Ultra-Large Bande dans les Réseaux Sans-fil” [Utilization of the Radiolocation Capabilities of Ultra-Wide Band Transmissions in Wireless Networks], Doctoral Thesis, Chapter 4, Section 4.4 “Positionnement, résolution ULB, et diversité temporelle—Application à la reconnaissance d'empreintes ULB en milieu indoor” [Positioning, UWB resolution, and temporal diversity—Application to the recognition of UWB imprints indoors”, pages 163-180, Institute National des Sciences Appliquées (INSA), Rennes, order No D05-18, Rennes, December 2005, document referenced [DOC 42] subsequently, advocates utilizing the result of the channel estimation phase directly as radio signature (for example the times of arrival of the most significant detected paths), for positioning purposes. However, this procedure requires a step of learning:                either on the basis of a very sizable campaign of measurements carried out at various geographical positions with a very fine spacing;        or else on the basis of an efficacious simulation tool; but in this case, it turns out to be difficult to predict waveforms realistically.        
The technique presented by J. M. Elwell et Al., “Systems and Method for Positioning using Multipath Signal”, in American patent application US 2008/0198072 A1 on 21 Aug. 2008, document referenced [DOC 21] subsequently, is a procedure for positioning and tracking mobile objects indoors, similar to the aforementioned fingerprinting techniques. This locating procedure utilizes the information arising from the UWB multi-path channel, namely measurements of the direct and secondary paths. The initial positioning of a mobile element is obtained, for example, by using a satellite-based positioning terminal. This initial position is thereafter associated with a multi-path profile received (solely by virtue of a filter) in order to couple this position to an initial configuration, stated otherwise a signature, of the UWB multi-path channel. Hereinafter, even if one or all the direct paths are lost (for example in a condition of obstruction of the radio linkups (NLOS), the filter thus constructed makes it possible to maintain an estimation of the position of the mobile element on the basis of the multi-path profile received.
However, this procedure requires an initial step where the mobile element determines its position accurately (by GPS). Moreover, each mobile element consists only of a single wireless node.
However, this locating procedure does not utilize any geometric relations between one or more of the secondary paths received and the position of one or more nodes attached to the mobile element (for example, the paths arising from reflections on the ground, on the wall or the ceiling). Furthermore, the locating procedure does not utilize the route related to the path reflected on the ground for the relative or absolute positioning and/or the tracking of the position of a wireless node. Moreover, this locating procedure considers only mobile elements or terminals carrying a single radio node.